Hydroformylation reaction strategies are widely known and have been well-documented over a span of many decades. Examples of literature that describes hydroformylation reactions include “New Syntheses with Carbon Monoxide”, Ed. J. Falbe, Springer Verlag, New York, 1980, especially the Chapter “Hydroformylation, Oxo Synthesis, Roelen Reaction” by B. Cornils; U.S. Pat. Nos. 3,527,809, 3, 917,661; 4,148,830; 4,742,178, 4,769, 984; 4,885,401; 6,049,011; 8,552,240; and U.S. Patent Pub. No. 2005/0065389, each of which is incorporated herein by reference in its respective entirety for all purposes.
Generally, hydroformylation involves reacting an unsaturated hydrocarbon with an excess of CO and hydrogen in the presence of a catalyst. Schematically, the reaction converts the double bond of the unsaturated hydrocarbon to a single bond, an H is added to the carbon atom on one side of the new single bond, and an aldehyde group, —C(O)H, is added to the carbon atom on the other side of the new single bond. Sometimes, small amounts of alcohol are also formed at this stage. Subsequent treatment techniques are used, as desired, to convert the aldehyde functionality into other functionality or to otherwise react the aldehyde with co-reactive functionality.
As a consequence of hydroformylation, an unsaturated compound containing “n” carbon atoms is converted into an aldehyde product containing “n+1” carbon atoms. For example, hydroformylation converts ethylene, a compound with 2 carbon atoms, into propionaldehyde, a compound with 3 carbon atoms. Hydroformylation causes the carbon chain to grow in length by one carbon atom. This is why hydroformylation has been described as a way to convert “lower” unsaturated hydrocarbons into a “higher” organic molecule.
Hydroformylation has been used to convert ethylene into propionaldehyde, which has then been converted into other C3 species such as propanol and propylene. This can be practiced using highly pure ethylene, e.g., from a mixture that is at least 99 weight percent ethylene. However, a substantial amount of commercially available ethylene is derived from feedstocks that, by their nature, contain both ethane and ethylene and thus are dilute in ethylene. Typically, such feedstocks result from processes in which the nature of the chemistry causes the feedstock to include no more than about 75 weight percent ethylene. It is difficult and expensive to purify these mixtures to obtain highly purified ethylene as might be desired for polyethylene manufacture due to the physical similarities between ethylene and ethane. Generally, using highly purified ethylene derived from such feedstocks is not an economically suitable way to practice hydroformylation of ethylene. This is particularly true in the case of shale gas recovery, where logistically isolated streams containing ethylene are expensive to purify due to the size of the resources and the lack of suitable pipelines.
Using the dilute ethylene mixtures also is problematic. Others have attempted this practice. See, e.g., U.S. Pat. No. 6,049,011. In these methods, the relatively expensive ethane is used to some degree to make ethylene, but overall the ethylene has low utilization. Impurities in the ethylene mixtures may also tend to poison and limit the expensive hydroformylation catalyst life/activity. In addition, other C3 and higher olefin impurities may lead to other undesirable by-products such as mixed aldehyde contaminates. The process also is inefficient in that relatively larger volumes of CO and hydrogen must be used to hydroformylate a dilute amount of ethylene effectively at an acceptable rate and degree of conversion. Also the excessive amounts of unreactive ethane and/or methane in the feed stream results in undesirably higher reaction pressures. This leads to relatively large purge streams leftover from the reaction that must be handled in some fashion. All of this adds substantial expense.
Consequently, even though hydroformylation techniques have been known and practiced for decades, the industry still needs a better way to practice hydroformylation of ethylene that is more efficient, more economical, and uses feed constituents with better utilization.